1. Technical Field
The present invention relates generally to rotor blade bearings for helicopters. More particularly, the present invention is directed to an elastomeric bearing for the flapping hinge of a helicopter.
2. Discussion
During horizontal flight of a helicopter, the difference in relative airspeed of the rotor blades causes an uneven lift between the blades. To compensate for this uneven lift, a flapping hinge is used to allow the rotor blades to vertically flap such that the rise or drop in a rotor blade compensates for the relative difference in lift between blades.
In the past, metal pin bearings have been used in the flapping hinge. These metal hinges suffer from high maintenance requirements and low bearing lives. Over the past 20 years, elastomeric bearings have become a worldwide industry standard in helicopter rotor systems. Elastomeric bearings provide greatly extended bearing lives while also reducing maintenance requirements.
A typical prior art elastomeric bearing generally includes a body with an elastomeric element therein, a hole through the center of the body for connection to the rotor hub, and a flange connected to the blades. Typically, installation of an elastomeric bearing includes assembling the rotor hub, taking a preload reading, disassembling the rotor hub, applying a preload and shim to the bearing, and reassembling the hub. This complicates installation and any maintenance that must be preformed on the bearing.
While prior art elastomeric bearings are useful, they are not without limitations. For example, the connection between an elastomeric bearing and the rotor hub typically involves only one, non-redundant, fastener. Failure of the single fastener can have catastrophic consequences on the helicopter. In military applications, this single fastener is of special concern wherein ballistic hits to the rotor hub are a possibility.
Accordingly, it is desirable to provide an elastomeric bearing that includes a redundant load path while also simplifying the installation process, thereby reducing installation and maintenance complexity.
In accordance with the teachings of the present invention, an elastomeric bearing for the flapping hinge of a helicopter is disclosed. The elastomeric bearing includes an outer race, an inner race, and an elastomeric element disposed therebetween. The outer race has at least one flange extending therefrom. A plurality of shear bolt holes are included in the flange such that a redundant load path is formed from one shear bolt hole to another. The elastomeric element is preloaded by applying a predetermined force to the outer race.